You can read anything from books like topics about your subject or do some research or even just learn new things from it. There lots of topics to read in books like english, math, science, poetry, history, art, designing, cooking, and lots more to see and read. Basically books are meant to acquire knowledge in facts and figures but some just tend to see pictures but don't read anything. Reading books might be a good way to expand your knowledge but never forget to learn from experience.
There are many types of reading materials aside from common books you usually see like encyclopedia, dictionary, thesaurus, magazines, pop-picture books, comics, novels, and many more but the one thing you can get from them which is the most important part is information.